There is considerable interest in the development of late transition metal complexes as catalysts for the polymerization of olefins. In particular, a variety of cationic and neutral group 10 complexes have proven effective for the production of branched poly(ethylene) from ethylene monomer and it is not necessary to add an α-olefin co-monomer so as to lower density with this general class of catalyst.
Branched poly(ethylene) is produced using these catalyst systems as chain-walking isomerization of the chain, involving a reversible β-H elimination/re-insertion sequence, competes with coordination (trapping) and insertion of monomer. This leads to linear low density polyethylene (LLDPE) with short chain branches or alternately to hyper-branched, amorphous polyethylene (PE) depending on the rates of insertion versus chain-walking.
As has been suggested, PE with long-chain branching as occurs in low density polyethylene (LPDE) or in ethylene-1-octene copolymers produced using Dow Chemicals Insite™ technology and which is desirable from the perspective of melt-processability is generally not available using late metal catalysts.
It is therefore of interest to note that the first preparation of branched PE from ethylene using the ill-defined catalyst reported by Keim et al. is said to provide a material similar to LDPE although no properties are reported. This catalyst formulation is generated in situ in the presence of ethylene through the reaction of either Ni(COD)2 or (π-allyl)2Ni with the sterically hindered phosphorane (Me3Si)2N—P(═NSiMe3)2 (Formula 1). In the case of (π-allyl)2Ni and phosphorane (Formula 1), the product of this reaction in the absence of monomer is shown to be a (π-allyl)Ni-iminophosphonamide (PN2) complex (Formula 2) in (Equation 1). The Pd-analog of (Formula 2) is structurally characterized but is inactive for ethylene polymerization. It is shown here that this complex (Formula 2) is also inactive in ethylene polymerization and thus the structure of the active catalyst in these original formulations is in doubt.

Subsequent work from the group of Yano in Japan established that Ni(COD)2 in combination with phosphorane (Formula 1) could be activated for ethylene polymerization using an α-olefin, and that the polymers contain Me and Hx+ branching in roughly equal amounts as judged from their 13C NMR spectra. Branches of intermediate length are not detected in the 13C NMR spectra while some of the longer branches present are of sufficient length to influence the hydrodynamic radius of the polymer in solution (i.e., g′=[η]br/[η]Bn=0.6 to 0.7). In a subsequent patent application, Yano et al. demonstrated that the reaction of e.g. Ni(acac)2 with phosphorane (Formula 1) gave rise to an active catalyst formulation in the presence of alkylaluminums, the polymers formed had similar properties.
The poly(ethylene) formed using the Keim family of catalysts is interesting from a materials perspective as it has variable crystallinity depending on both molecular weight and branching but should process similar to low-density PE. However, the activity and stability of these catalyst formulations is too low for practical use.